Refactor translation editor save logic to improve focus handling and save timing

[meel-hd]

Fixes:

• Stop save triggered when clicking source text or inserting placeholders.
• Cancel save operation after (1000ms) quick refocus
• Reduce possibility of multiple validation popups showing in the UI because of unintended saves

Close: #3410

[nijel]

Good point, let's deal with that separately - https://github.com/WeblateOrg/weblate/issues/15261.